Support sub-daily granularity in YAML configs

[courtneyholcomb]

Resolves #818

Description

Enable sub-daily granularities in YAML specs (for time dimensions & cumulative metric windows).

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have run changie new to create a changelog entry